static inline size_t sdslen(const sds s) {
    unsigned char flags = s[-1];
    switch(flags&SDS_TYPE_MASK) {
        case SDS_TYPE_5:
            return SDS_TYPE_5_LEN(flags);
        case SDS_TYPE_8:
            return SDS_HDR(8,s)->len;
        case SDS_TYPE_16:
            return SDS_HDR(16,s)->len;
        case SDS_TYPE_32:
            return SDS_HDR(32,s)->len;
        case SDS_TYPE_64:
            return SDS_HDR(64,s)->len;
    }
    return 0;
}

sds sdscpylen(sds s, const char *t, size_t len) {
    // 判斷字符串數組分配的空間長度是不是小于字符串數組當前長度
    if (sdsalloc(s) < len) {
        // 根據要追加的長度len-sdslen(s)和現有長度，判斷是否增加新的空間
        s = sdsMakeRoomFor(s,len-sdslen(s));
        if (s == NULL) return NULL;
    }
    // 將源字符串t中len長度的數據拷貝到目標字符串結尾
    memcpy(s, t, len);
    // 拷貝完后，在目標字符串結尾加上\0
    s[len] = '\0';
    // 設置字符串數組最新當前長度
    sdssetlen(s, len);
    return s;
}

sds sdsMakeRoomFor(sds s, size_t addlen) {
    void *sh, *newsh;
    // 獲取sds可用空間
    size_t avail = sdsavail(s);
    size_t len, newlen;
    char type, oldtype = s[-1] & SDS_TYPE_MASK;
    int hdrlen;
 
    // 如果可用空間大于等于要增加的空間，則直接返回
    if (avail >= addlen) return s;
    // sds長度
    len = sdslen(s);
    // sds指針
    sh = (char*)s-sdsHdrSize(oldtype);
    // 新字符串長度
    newlen = (len+addlen);
    // 如果新長度小于最大預分配長度，則進行兩倍擴容
    if (newlen < SDS_MAX_PREALLOC)
        newlen *= 2;
    else
        newlen += SDS_MAX_PREALLOC;
    type = sdsReqType(newlen);
    // SDS類型5轉換為類型8
    if (type == SDS_TYPE_5) type = SDS_TYPE_8;
 
    hdrlen = sdsHdrSize(type);
    if (oldtype==type) {
        newsh = s_realloc(sh, hdrlen+newlen+1);
        if (newsh == NULL) return NULL;
        s = (char*)newsh+hdrlen;
    } else {
        /* Since the header size changes, need to move the string forward,
         * and can't use realloc */
        newsh = s_malloc(hdrlen+newlen+1);
        if (newsh == NULL) return NULL;
        memcpy((char*)newsh+hdrlen, s, len+1);
        s_free(sh);
        s = (char*)newsh+hdrlen;
        s[-1] = type;
        sdssetlen(s, len);
    }
    sdssetalloc(s, newlen);
    return s;
}

struct __attribute__ ((__packed__)) sdshdr5 {
    unsigned char flags; /* 3 lsb of type, and 5 msb of string length */
    char buf[];
};
struct __attribute__ ((__packed__)) sdshdr8 {
    uint8_t len; /* used */
    uint8_t alloc; /* excluding the header and null terminator */
    unsigned char flags; /* 3 lsb of type, 5 unused bits */
    char buf[];
};
struct __attribute__ ((__packed__)) sdshdr16 {
    uint16_t len; /* used */
    uint16_t alloc; /* excluding the header and null terminator */
    unsigned char flags; /* 3 lsb of type, 5 unused bits */
    char buf[];
};
struct __attribute__ ((__packed__)) sdshdr32 {
    uint32_t len; /* used */
    uint32_t alloc; /* excluding the header and null terminator */
    unsigned char flags; /* 3 lsb of type, 5 unused bits */
    char buf[];
};
struct __attribute__ ((__packed__)) sdshdr64 {
    uint64_t len; /* used */
    uint64_t alloc; /* excluding the header and null terminator */
    unsigned char flags; /* 3 lsb of type, 5 unused bits */
    char buf[];
};